Selected-sequence biopolymers, such as peptides and oligonucleotides, are routinely synthesized by solid-phase methods in which a series of selected polymer subunits are added sequentially to a growing polymer chain covalently attached to a solid support. In a typical peptide synthesis, the peptide is synthesized stepwise from an immobilized C-terminal residue. At each step, a new N-protected amino acid is added in solution to the solid support, and reacted through its free carboxyl group with the free .alpha.-amine group of the amino acid (or peptide) immobilized on the support, to couple the new amino acid to the growing peptide on the support. The support is then treated to remove the N-protecting group of the last added amino acid, and the procedure is repeated in a stepwise fashion until the final peptide is complete.
An oligonucleotide, e.g., DNA strand, is similarly synthesized by solid-phase methods, by stepwise addition of a selected 5'-protected nucleotide to a resin containing an immobilized 3'-end nucleoside Backbone coupling is between the free 5' OH group of the immobilized nucleoside, and the activated 3'-end of the free nucleotide. After the coupling reaction, the support is treated to remove the 5'-end protecting group, and the reaction steps are repeated stepwise until the desired-sequence polynucleotide is complete.
Solid-phase methods for peptide and oligonucleotide synthesis can be carried out conveniently by automated synthesizers which are designed for successive addition of selected subunits, coupling agents, and deprotecting agents to a vessel containing the solid-phase material. That is, each subunit addition step involves (a) adding a deprotection solution to the solid-phase vessel, to deprotect the last-added residue on the immobilized support, and (b) adding the next subunit, either in activated form or in the presence of an activator, to the solid-phase vessel, to couple the subunit to the growing polymer chain on the solid support.
In a typical operation of an automated synthesizer, the machine is first loaded with vials containing each of the subunits which are to be added during operation, and with the deprotection and wash solutions used during operation. The vials containing the individual subunits may be prepackaged in liquid form, allowing the subunit solution to be transferred readily from the vial to the solid-phase vessel. Such vials, of course, must be stored in a manner which prevents leakage or breakdown of the subunit or activation components.
Alternatively, the subunit materials may be packaged in the vials in dry form. The dried material is either manually dissolved prior to loading into the machine, or more commonly, is dissolved during machine operation, by addition of a selected volume of solvent to each vial. One limitation of the dry material is that, for many protected amino acids, poor powder flow properties and low bulk density of the material makes the material difficult to measure and package. In addition, some solid protected amino acids dissolve very slowly, requiring up to thirty minutes of contact time with an added solvent before the subunit is fully dissolved. For these reasons, a composition having handling and dissolution properties which are superior to those of the existing available reagents would be useful.